Assessing disease activity in people with Takayasu's arteritis is difficult, as most people with the disease do not exhibit visible, measurable symptoms. Combination positron emission tomography/computed axial tomography (PET/CT) is a particularly sensitive, accurate scanning method that may improve researchers' ability to evaluate people with the disease. This study will determine the effectiveness of PET/CT in assessing disease activity in people with Takayasu's arteritis.